The presentation of three distinct Neidium taxa from the sub-Antarctic and Maritime Antarctic region after examining more than 1500 samples from a broad variety of aquatic and terrestrial (micro-)habitats suggests that taxa representing the genus Neidium Pfitzer are not only quite rare in these regions, but that the three species also represent well-defined biogeographical distributions. N. nyvltii sp. nov., N. antarcticum sp. nov. and N. aubertii Manguin are characterized by linear to linearlanceolate valves with rostrate to capitate apices, a single prominent longitudinal canal along the valve margins and lacinia covering the distal raphe endings. Neidium nyvltii is a benthic alkalophile growing in the Maritime Antarctic region under high total phosphorus (TP) and moderately high chloride concentrations, while the closest taxa for comparison (N. kozlowii Mereschkowsky and its varieties) are also alkalophilic, growing under lower TP concentrations in northern temperateArctic regions. Neidium aubertii is an acidophile with a sub-Antarctic distribution, whereas the most comparable taxa are N. bisulcatum (Lagestedt) Cleve, and N. bergii (A. Cleve-Euler) Krammer & Lange-Bertalot both are alkalophiles with northern hemisphere distribution and commonly occurring in the Arctic. There is some evidence of parallel similarities in valve forms between the polar northern and southern hemispheres. The significance of this parallelism is yet to be determined.